Hi sorry I didn't see your reply to an old post.
1. error - probably not a problem. The reason there is no disParticipant list is that the list isn't created until the first front end login, it seems, according to the info of squarou recently.
2. This only displays site homepage issue maybe can be fixed by reading this post: http://forums.modx.com/thread/89865/getting-started-htaccess-w-code
Wait, when you say site homepage, you mean the forum front page, all forums? If its showing your default page instead, then that seems to be an error and you have it set to go to that page on error, I would think.
If this is the case then something isn't letting it display, presumably not related to the user class. There could be a lot of reasons so you would have to go through it all again.
3. groups error - hmm thats a new one. Did you try ginzola1's fix? What do you see when clicking on user groups tab? Can you see the forum members and forum moderators listed there? Try using those and see what happens.
This function doesn't work very well even in the best case. Some notes:
I just posted what I did to define and show user classes in the post view and in the profile view with custom php code
I was able to make custom classes through the manager, but they are basically meaningless as they don't seem to really do anything
Mods can be set in the system settings, not just in the dialog you are looking at
System setting of 'use custom classes' may or may not be relevant
Setting up permissions for individual threads and so on is a pain, not sure how well it works, even if you can set up custom classes
So all of that is sort of saying, you don't need the manager for this function for the most part.
4. Don't forget to go through system settings to get things working, and publish the page that the forum is in.
Finally, given that it might be an sql error, could you have an old version of mysql? Dommes89 reported this error but I know he got it running eventually.